1. Field of the Invention
The invention relates to the optical power regulators for fiber optical network, and particularly to a variable optical attenuator (VOA) for controllably varying the strength of the optical signals.
2. The Related Arts
In the fiber optic networking system, the light signals are transited along optical fibers to transfer information from one location to another. It is often desirable to tailor the power of the optical signals within optical fiber networks. For example, the individual components of an optical fiber network may be tested by using a low power optical signal to simulate fiber optic telecommunications or data communications over a long distance. Tailoring of optical signal strengths is expected in automatic optical testing systems, optical signal routing systems, and optical sensor arrays etc.
U.S. Pat. No. 5,3111,613 issued on Oct. 5, 1999, describes an attenuator based on polarization modulator (a liquid crystal material). This type attenuator has an issue of controlling PDL (polarization dependent loss), and the associated optical performance is subject to the environmental temperature.
U.S. Pat. No. 5,745,634 issued Apr. 28, 1998, describes a voltage-controlled attenuator where the optical path consists of two collimators, one mirror and one filter. The driving means of the filter is constructed by a DC motor, a gearbox, and a rotation disk. There is a lever labeled 30 in FIG. 2B thereof which connects the filter and the rotation disk. This mechanical structure has a concern of difficulties of making the VOA device compact since the optical path of the optical components is essentially parallel to the mechanical structures. Also, the cost of this device is relatively high since the precision gearbox is utilized thereof for implementation of the whole design.
U.S. Pat. No. 6,130,984 issued Oct. 10, 2000, discloses a voltage controlled VOA device. A dual fiber collimator is used. In terms of the topical performance, it has a concern of the PDL. Also, the filter driving means, i.e., the motor, is arranged perpendicular to the direction of the fiber or the optical path. This type design results in irregular package size. As illustrated in column 6, lines 5-10 thereof, the device length along the fiber direction is 37 mm, but the device width perpendicular to that fiber direction, is 70 mm. It means that the width is much longer than the length. In respect with the optical system design, there is a desire to have a smaller device width for the compact system equipment. U.S. Pat. No. 6,292,616 demonstrates a U-like frame to form an optical path where the ND filter is designed to be wedge-shaped to reduce the wavelength sensitivity. However this device is subject to the relatively large PDL thus being unsuitable for the high-speed optical network. In general, the PDL requires  less than 0.1 dB for the data transfer rate of 10 Gb/s and 0.05 dB for the data transfer rate of 40 Gb/s. Also, such a device lacks the filter position indicator to precisely monitor the desired position of the ND filter. Thus, the user can not dynamically control the optical attenuation setting. It is hardly integrated into the practical optical network management.
In light of the foregoing, it would be desirable to provide improved structures and methods for attenuating optical signals, and particularly with low cost and high reliabilities.
According to an aspect of the invention, a controllable attenuator comprises a first collimator for receiving the incoming light beam and directing the light beam along a first beam path. A first 45xc2x0 mirror changes the light beam by 90xc2x0, and a second 45xc2x0 mirror successively changes the light beam by another 90xc2x0. Then, the light beam enters the second collimator and leaves therefrom via a fiber. A controllable attenuating device, i.e., the associated a ND filter, is located between the first mirror and the second mirror, and is generally laterally moveable in the optical path for varying the attenuation of the outing light beam.
Yet, a miniature driving means of the ND filter is provided with a stepping motor connected to a lead screw for driving the ND filter, and a potentiometer place under the ND filter to read the precise position of the ND filter.
Advantageously according to the present invention the attenuation can be continuously adjusted over a broad range. The electrical signal from the potentiometer gives a feedback to dynamically tune the attenuation. Also, the whole device is well compacted via unique combination of the optical components and the electrical and mechanical driving device.